A traditional camera system includes a camera body, lens, electronics to control the functionality of the camera, and the image-capturing media, film. In contrast, instead of using film, a digital camera usually employs an image sensor made on semiconductor substrate. Typically the image sensor is either a Charge Coupled Device (CCD) or a Complementary Metal Oxide Semiconductor (CMOS) image sensor. For illustrative purposes only, the image sensor discussed herein is an area CCD image sensor.
An area CCD image sensor consists of a number of photosensitive elements called pixels that are arranged in rows and columns to form a two-dimensional imaging array. The pixels capture light from a scene of interest and convert the incident light into charge carriers. The charge carriers are then transferred out of the imaging array using vertical CCD shift registers and one or more horizontal CCD shift registers. Each CCD shift register has at least one output structure which consists of one or more transistors, namely amplifiers, to convert the charge carriers from the pixels to voltage signals. The voltage signals are then converted to digital signals and the digital signals processed to render a digital image.
An amplifier typically consists of one or more transistors, each of which has a drain, a source and a gate. Referring to FIG. 1, there is shown a cross-section of a transistor 100 built on a semiconductor layer 102. It is noted that other types of transistors are applicable to the present invention, in which case the doping will vary according, as those skilled in the art will readily recognize. Source/drain regions 104, 106 of transistor 100 consist of n-type doping regions that are connected to contacts 108, 110.
Source/drain 106 is usually connected to a high voltage source (VS) through contact 110. The voltage applied to source/drain 106 can create a strong electric field between gate 112 and source/drain 106 that generates a near-infra-read (NIR) light. The NIR light is mainly generated along the gate-source/drain side 114 of transistor 100 and propagates in every direction as indicated by the arrows. There are two light propagation paths that can potentially impact the imaging array.
One light propagation path 116 is the path through which light passes out of transistor 100 and then bounces back into a pixel within the imaging array. Another path 120 has the light passing through and underneath surface 118 and within the semiconductor layer 102, where it propagates into the imaging array. Path 120 has another potential impact. The charge carriers 122, such as electrons (e) or holes (p) that are generated by the light as it propagates along path 120, can diffuse into the imaging array.
When the light propagating along paths 116 and 120 enters the imaging array of the image sensor, it causes the affected pixels to generate additional charge carriers. These additional charges will be superimposed on the existing charge carriers related to the captured image, thereby creating a glowing phenomenon or artifact in the captured image. The glowing artifact is usually seen in one of the corners of the imaging area that is near the location of the amplifier.
FIG. 2 is a simplified diagram of a CCD image sensor that has an amplifier at the lower-right corner in an embodiment in accordance with the prior art. When a captured image is read out of imaging array 200, charge carriers are simultaneously transferred through each vertical CCD shift register 202 on a row-by-row basis to horizontal shift register 204. The vertical transfer direction is indicated by arrow 206.
After each row is transferred to horizontal shift register 204, the charge carriers are serially transferred through horizontal shift register 204 to output amplifier 208. The horizontal transfer direction is indicated by arrow 210. The light emitting from amplifier 208 enters the pixels positioned at the lower-right corner of imaging array 200 to create an amplifier glow artifact in region 212 of imaging array 200.
Since amplifier 208 only glows when the voltage is applied to its source/drain region (e.g., region 106 in FIG. 1), one prior art solution to reduce amplifier glow is to minimize the amount of time output amplifier 208 is operational. Thus, during the image exposure period for imaging array 200, the voltage source is set to zero in order to turn off amplifier 208. This method removes the glow during the image exposure period, but amplifier 208 must be turned on during image readout. And during image readout, amplifier glow occurs and affects the images captured by image sensor 214. Therefore, the impact from amplifier glow cannot be completely removed by this prior art method.
U.S. Pat. No. 7,402,882 discloses another technique for reducing amplifier glow. Multiple opaque layers are disposed around an amplifier to form an encapsulation for the amplifier. The encapsulation prevents the NIR light, and the charges generated by the NIR light, from affecting adjacent pixels. One possible limitation to this technique is the number of opaque layers used to form the encapsulation. The multiple layers, and in particular the layer disposed over the amplifier, adds complexity to the fabrication process for the image sensor. The additional complexity increases the cost of fabricating an image sensor.